On cigarette packing machines, a group of cigarettes is first formed, normally comprising three superimposed layers of 7-6-7 cigarettes respectively, and which is fed to a succession of packing wheels. Before performing any of the packing operations, however, it is common practice to perform on each group of cigarettes a series of checks, including detecting the presence of all the cigarettes in the group.
This is known to be done using a device of the type described in U.S. Pat. No. 5,235,649 (or in the corresponding EP Patent No. 518,141), which employs an optical-fiber bundle, wherein a first end of each optical fiber is so positioned as to face, at a control station, the tip of a respective cigarette in the group for examination, and a second end of each optical fiber is connected optically to the lens of a CCD television in turn connected to a digitizing board of a monitoring computer.
In actual use, each group of cigarettes is fed into the control station, where the camera picks up an image of the tips on one side of the group; the digitizing board connected to the camera digitizes the image; and the computer acquires the digitized image from the digitizing board in the form of a matrix of brightness values, and processes the image to determine the presence or absence, and possible also the fill density, of each cigarette.
Devices of the above type are fairly expensive, by requiring a television camera with a respective optical assembly, a digitizing board, and a monitoring computer, and are even more expensive when high-speed operation is called for, as in the case of modern cigarette packing machines capable of producing as many as ten packets of cigarettes a second. Consequently, known devices of the above type are normally only justified when used to determine both the presence and the tip fill density of the cigarettes, and are redundant when used for presence detection only, by acquiring and processing a "complex" image, i.e. containing information which is of no use for presence detection purposes only.
U.S. Pat. No. 5,299,275 discloses a device comprising a boundle of optical fibers, which is used as a blur filter to limit high spatial frequencies incident upon an image sensor. Each optical fiber has a first end coupled to a lens and a second end coupled directly to a photosensitive surface of a CCD sensor. In the above known device, in order to obtain a blur filtering effect, the second end of each optical fiber is maintained separated by a given distance from the photosensitive surface of the CCD sensor.